The present invention relates to an enciphered facsimile apparatus, and more particularly to an enciphered facsimile apparatus for transmitting or receiving image information enciphered by a chained enciphering method.
Conventionally, in a case where an attempt is made to effect image communication by facsimile without letting the communication known to a third party, the communication is effected by enciphering image data. There are various methods of enciphering, and a chained enciphering method is one of them.
As for the basic principle of this enciphering method, in a case where a plain text (an original text to be enciphered) is comprised of elements X1, X2, X3, . . . , as shown in FIG. 7, the first element X1 and arbitrary key data are exclusively ORed to determined a first encipher text element Y1, and then the second element X2 and the enciphered text element Y1 are exclusively ORed to create a second enciphered text element Y2. In the same way thereafter, the third element X3 of the original text and the enciphered text element Y2 are exclusively ORed to create a third enciphered text element Y3.
To decipher the enciphered text, as shown in FIG. 8, the first enciphered text element Y1 and the key data are exclusively ORed to obtain the first element X1 of the original text. Then, the second enciphered text element Y2 and the enciphered text element Y1 are exclusively ORed to obtain the second original text element X2. Thereafter, deciphering is continued by similar processing.
Concerning the above-described encipherment, a disclosure is given in "Block Chain Using Feedback of Plain Text and Enciphered Text" on pages 71-75 in a publication "Cipher: New Development of Protection of Computer Data" published by Kabushiki Kaisha Shizensha on Feb. 10, 1986.
In effecting confidential communication, the transmission-side facsimile apparatus first encodes image data read by an image scanner or the like, subsequently enciphers the same by the above-described method, and then modulates the enciphered data and transmits the same to a line. The reception-side facsimile apparatus demodulates the data received, and after deciphering the enciphered text by the above-described deciphering method, the reception-side facsimile apparatus decodes the same. Then, this decoded text is sent to a printer so as to be printed out.
In this case, since the method of sequentially deciphering the image data beginning with its leading end is adapted in the transmission-side facsimile apparatus, the reception-side facsimile apparatus sequentially deciphers the image data received beginning with its leading end according to the conventional method.
In the communication of an image using a facsimile apparatus, the deciphering of the image data enciphered by the above-described enciphering method has hitherto been effected starting with the leading end of the image data. Therefore, firstly, the starting point of commencing deciphering is obvious to anyone, and, secondly, since the image data deciphered is sequentially decoded beginning with the leading end thereof, it is possible to easily confirm whether or not deciphering has been effected correctly. For these reasons, there has been the problem that it is relatively easy for a third party to effect deciphering.
In Addition, in a case where image data enciphered by the above-described chained enciphering method is transmitted, if a data error occurs in the course of transmission, it becomes impossible to decode the subsequent data. Hence, the facsimile apparatus adopting the enciphering method has had the problem that it is vulnerable to noise.